Memory device and method

ABSTRACT

During first portion of a first read cycle determining that a first input of a sense amplifier is to receive information based upon a state of a storage cell during a first portion of a read cycle, and determining that a conductance at the first input is substantially equal to a conductance at a second input of the sense amplifier during the first portion. A plurality of NAND string modules are connected to a global bit line of a memory device that includes a memory column where a plurality of NAND strings and a buffer are formed.

CROSS-REFERENCE TO REPLATED APPLICATIONS

The present disclosure is related to U.S. patent application Ser. No.______ (Attorney Docket No. AF02932), filed on an even date herewith andentitled “MEMORY DEVICE AND METHOD,” and U.S. patent application Ser.No. ______ (Attorney Docket No. AF02933), filed on an even date herewithand entitled “MEMORY DEVICE AND METHOD,” the entirety of which isincorporated by reference herein.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates generally to memory devices and morespecifically to a non-volatile memory device.

2. Description of the Related Art

Current flash memories, such as NAND-type flash memories, are notscalable to Giga-bit densities without a substantial loss ofperformance. A flash memory device capable of high-speed and highdensities would be useful.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerousfeatures and advantages made apparent to those skilled in the art byreferencing the accompanying drawings.

FIG. 1 illustrates a block diagram of memory device in accordance with aspecific embodiment of the present disclosure.

FIG. 2 illustrates a block diagram of a sense amplifier in accordancewith a specific embodiment of the present disclosure.

FIG. 3 illustrates a circuit diagram of a portion of the sense amplifierof FIG. 2 in accordance with a specific embodiment of the presentdisclosure.

FIG. 4 illustrates a timing diagram associated with the portion of thesense amplifier of FIG. 3 where a non-conductive memory cell is beingread.

FIG. 5 illustrates a timing diagram associated with the portion of thesense amplifier of FIG. 3 where a conductive memory cell is being read.

FIG. 6 illustrates a method in accordance with a specific embodiment ofthe present disclosure.

FIG. 7 illustrates a method in accordance with a specific embodiment ofthe present disclosure.

FIG. 8 illustrates a circuit diagram of a portion of the sense amplifierof FIG. 2 in accordance with a specific embodiment of the presentdisclosure.

FIG. 9 illustrates NAND string modules associated with the memory deviceof FIG. 1 in accordance with a specific embodiment of the presentdisclosure.

FIG. 10 illustrates a NAND string module in greater detail in accordancewith a specific embodiment of the present disclosure.

FIG. 11 illustrates a portion of FIG. 10 in greater detail in accordancewith a specific embodiment of the present disclosure.

FIG. 12 illustrates a portion of FIG. 11 in greater detail in accordancewith a specific embodiment of the present disclosure.

FIG. 13 illustrates the NAND strings of FIG. 12 in greater detail inaccordance with a specific embodiment of the present disclosure.

FIG. 14 illustrates a portion of FIG. 10 in greater detail in accordancewith a specific embodiment of the present disclosure.

FIG. 15 illustrates a portion of a NAND string module of FIG. 10 incircuit diagram form using transistors formed at memory array columns inaccordance with a specific embodiment of the present disclosure.

FIGS. 16-20 illustrate physical layers in stick diagram form of thecircuit diagram of FIG. 15 in accordance with a specific embodiment ofthe present disclosure.

FIG. 21 illustrates a method in accordance with a specific embodiment ofthe present disclosure.

FIG. 22 illustrates a method in accordance with a specific embodiment ofthe present disclosure.

FIG. 23 illustrates a method in accordance with a specific embodiment ofthe present disclosure.

FIGS. 24-29 illustrates cross sectional views corresponding to variouslocations of FIGS. 16-20.

FIG. 30 illustrates a sense amplifier in accordance with a specificembodiment of the present disclosure.

FIG. 31 illustrates a specific layout of global bit lines from a planview in accordance with a specific embodiment of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates a block diagram of a memory device 10 in accordancewith a specific embodiment of the present disclosure. Memory device 10is illustrated to include a memory array 12, a sense amplifier module17, and a decode module 18. The memory array 12 is connected to thesense amplifier module 17 to receive information by global bit lines131-136. The memory array 12 is connected to the decode module 18 toreceive information by connections 186. The sense amplifier module 17 isconnected to the decode module 18 to receive information by connections187.

Sense amplifier module 17 includes sense amplifiers 14-16 that providedata bits D0-Dz, where “z” indicates a number of sense amplifiersaccessing data. In one embodiment of the present disclosure, each senseamplifier of sense amplifiers 14-16 provides one logic value at itsoutput when it determines a common conductance is present at its inputsduring a read cycle, and provides a different logic value at its outputwhen it determines there is not a common conductance present at itsinputs during the read cycle. For purposes of discussion herein, thememory device 10 is primarily described with respect to a read cyclethat accesses information stored at a storage cell of the memory device16. Therefore, it will be appreciated that inputs to some devicesdescribed herein, such as sense amplifier 14, with respect to a readoperation can also function as outputs during a write operation, andthat outputs of some devices during a read operation can also functionas input during a write operation.

Memory array 12 includes a plurality of sub arrays 121-126 thatrespectively include a plurality of memory array columns. Each sub arrayof the plurality of sub arrays 121-126 is respectively connected to acorresponding global bit line of the global bit lines 131-136. Each subarray of the plurality of sub arrays 121-126 respectively includes acorresponding plurality of storage elements, also referred to as storagecells, that store information to be communicated to the sub array'scorresponding global bit line. In one embodiment of the presentdisclosure, the sense amplifiers 14-16 determine a storage state of astorage element, which can be volatile or non-volatile, based upon arelative conductance at a first input and at a second input of the senseamplifier. In another embodiment of the present disclosure, eachrespective storage element of memory array 12 is non-volatile andassociated with a corresponding NAND string, and the sense amplifiers14-16 can detect a relative conductance or other characteristics attheir inputs indicative of a storage state of the storage element beingread.

During normal operation, the decode module 18 provides control signalsto the memory array 12 via interconnects 186 to the sense amplifiermodule 17 via interconnects 187 as described herein to access specificstorage elements of memory array 12. Various aspects of the memorydevice 10 will be better understood with reference to FIGS. 2-7.

FIG. 2 illustrates a block diagram of specific embodiment of senseamplifier 14 of FIG. 1. The sense amplifier 14 includes amplifier 141,latch 143, a sense node 148 also labeled SNw, a sense node 149 alsolabeled SNe, current source 144 to provide a reference currentIw_(sense), and current source 145 to provide reference currentIe_(sense). Amplifier 141 is illustrated to include a pre-charge module1412. It will be appreciated that each of the sense amplifiers of senseamplifier module 17 will typically be the same as sense amplifier 14.

The sense amplifier 14 is connected to the memory array 12 at an inputof amplifier 141 labeled INw that is connected to interconnect 131,which is a global bit line also labeled GBLw, and at an input labeledINe connected to interconnect 132 that is a global bit line also labeledGBLe. The amplifier 141 is connected to the decode module 18 by variousinterconnects of interconnects 187 including: an interconnect labeledAMPLIFY; an interconnect labeled AMPLIFYB; an interconnect labeledVt_CANCEL; an interconnect labeled READw; an interconnect labeled READe;and an interconnect labeled PC_GBL. These interconnects are associatedwith reading data from memory array 12, and providing a differentialsignal at the sense nodes SNw and SNe.

The latch 143 is formed by six PFETs, illustrated with a diagonalindicator at their respective sources, and thirteen NFETs. Latch 143 isconnected to the sense nodes SNw and SNe and to the decode module 18 byan interconnect labeled LAT, of interconnects 187. Data from the latch143 is provided to an output of the sense amplifier 14 labeled DATA.Operation of sense amplifier 14 of FIG. 2 will be better understood withreference to FIGS. 3-7 herein.

FIG. 3 illustrates global bit line GBLw, global bit line GBLe, N-typefield effect transistors (FETs) 1491 and 1492, resistive element 1493,storage cell 1494, and a circuit representing a more detailed view of aportion 1411 and including pre-charge portion 1412 of the amplifier 141of FIG. 2.

The pre-charge portion 1412 of amplifier 141 includes N-type FETs1461-1463. Portion 1411 of amplifier 141 implements a differentialamplifier during operation and includes: N-type FETs 1410, 1420,1430-1433, and 1440-1443; and capacitors 1436, 1437, 1446, and 1447.

Transistor 1410 includes a control electrode, e.g. a gate, connected toa node labeled AMPLIFYB, a first current electrode, e.g. a source/drainelectrode, and a second current electrode connected to INw. Transistor1430 includes a control electrode, e.g. a gate, connected to a nodelabeled AMPLIFY, a first current electrode, e.g., a source or drain,connected to the first current electrode of transistor 1410, and asecond current electrode. Transistor 1431 includes a control electrodeconnected to a node labeled Vt_CANCEL, a first current electrodeconnected to the second current electrode of transistor 1430, and asecond current electrode. Transistor 1435 includes a control electrode,a first current electrode connected to the first current electrode oftransistor 1431, and a second current electrode connected to the secondcurrent electrode of transistor 1410. Transistor 1432 includes a controlelectrode connected to a node labeled Vt_CANCEL, a first currentelectrode connected to the second current electrode of transistor 1435,and a second current electrode. Transistor 1433 includes a controlelectrode connected to a node labeled READw, a first current electrodeconnected to the second current electrode of transistor 1432, and asecond current electrode. Capacitor 1436 includes a first electrodeconnected to the second current electrode of transistor 1432, and asecond electrode connected to the second current electrode of transistor1435. Capacitor 1437 includes a first electrode connected to the secondcurrent electrode of transistor 1433, and a second electrode connectedto the second current electrode of transistor 1435.

Transistor 1420 includes a control electrode connected to a node labeledAMPLIFYB, a first current electrode, and a second current electrodeconnected to INe. Transistor 1440 includes a control electrode connectedto a node labeled AMPLIFY, a first current electrode connected to thefirst current electrode of transistor 1420, and a second currentelectrode. Transistor 1441 includes a control electrode connected to anode labeled Vt_CANCEL, a first current electrode connected to thesecond current electrode of transistor 1440, and a second currentelectrode. Transistor 1445 includes a control electrode connected to thefirst electrode of capacitor 1437, a first current electrode connectedto the first current electrode of transistor 1441, and a second currentelectrode connected to the second current electrode of transistor 1420.Transistor 1442 includes a control electrode connected to the nodelabeled Vt_CANCEL, a first current electrode connected to the secondcurrent electrode of transistor 1445, and a second current electrode.Transistor 1443 includes a control electrode connected to the nodelabeled READe, a first current electrode connected to the second currentelectrode of transistor 1442, and a second current electrode connectedto the control electrode of transistor 1435. Capacitor 1446 includes afirst electrode connected to the second current electrode of transistor1442, and a second electrode connected to the second current electrodeof transistor 1445. Capacitor 1447 includes a first electrode connectedto the second current electrode of transistor 1443, and a secondelectrode connected to the second current electrode of transistor 1445.

Transistors 1410, 1430-1433, 1420, 1440-1443, and 1461-1463 are alsoreferred to as switches, in that they are controlled by logic signalsthat result in their operating effectively as a switch that is open orclosed to allow or prevent information from being communicated therethrough.

Transistor 1491 operates as a buffer in a sense path of memory device 10that also includes a storage cell 1492 that is being read. The controlelectrode of transistor 1491 is biased at reference voltage V_(PWR)during a portion of a read cycle if the storage cell being read is in anon-conductive state, and will be biased at a lower voltage, such as atreference voltage near V_(GND), during the portion of the read cycle ifthe storage cell being read is in a conductive state. The source oftransistor 1491 is connected to V_(GND). The drain of transistor 1491 isconnected to GBLw. Transistor 1491 is configured as an open drainbuffer, whereby transistor 1491 is turned off in response to V_(GND)being applied to its control gate, which results in the output oftransistor 1491 communicating a conductive state of a storage cell 1494when being read, by generating a high-impedance at its output, e.g., thedrain of transistor 1491 is isolated from its source. Transistor 1491 isturned on in response to V_(PWR) being applied to its control gate,which results in the output of transistor 1491 communicating anon-conductive state of a storage cell being read by generating alow-impedance at its output, e.g., charge at GBLw is allowed to flowfrom the output of transistor 1491, i.e., its drain, to the interconnectV_(GND).

Storage cell 1494 can be a NAND storage cell of a NAND string, and isillustrated as a transistor that includes a control electrode, a firstcurrent electrode, and a second current electrode. A charge can bestored at the storage cell 1494 causing it to be more or less conductivein response to a read bias signal being asserted at its controlelectrode during a read operation. For purposes of discussion withrespect to FIG. 3, the storage cell 1494 is being read, and thereforethe control gate is receiving an asserted read bias signal at node WL. Asource electrode of storage cell 1494 is illustrated as being connectedto a node at a ground potential, and a drain electrode of storage cell1494 is illustrated as being connected to the control gate of transistor1491. In response to the bias signal being asserted at node WL whenstorage cell 1494 is conductive, the voltage V_(DATA) will be less thanthe reference voltage provided to node V_(PWR). For example, V_(DATA)may be near V_(GND). In response to the bias signal being asserted atnode WL when storage cell 1494 is non-conductive, the voltage, V_(DATA),will be at the reference voltage V_(PWR).

FIG. 4 illustrates a timing diagram for a read cycle during whichstorage cell 1494 is being read while in a conductive state. Forpurposes of discussion, a read cycle is described from time 916 of RC1to time 916 or RC2, which includes portions of two formal read cycles,RC1 and RC2. At time portion 923 of the read cycle RC1, signals PC_GBL,represented by timing trace 931, VT_CANCEL, represented by timing trace932, and AMPLIFY, represented by timing trace 934, are asserted.

In response to PC_GBL being asserted at time 916 of RC1, a definedvoltage, V_(EQ), is provided to both GBLw and GBLe, resulting in bothGBLw and GBLe being driven to V_(EQ) at time period 923 of RC1. V_(EQ)is also referred to as a pre-charge voltage in that the global bit linesGBLw and GBLe are pre-charged to this voltage. Therefore, becausetransistors 1461 and 1462 can be simultaneously closed, the bottomelectrode of transistor 1447 is selectively communicatively connected tothe input INw, and the bottom electrode of transistor 1437 isselectively communicatively connected to the input INe.

As described above, the control electrode of transistor 1435 isselectively communicatively connected to its drain by transistor 1431.For example, in response to Vt_CANCEL being asserted at time 916 of RC1,switch 1431, is closed to connect the gate and drain of transistor 1435together, and switch 1441 of amplifier 141 is closed to connect the gateand drain of transistor 1445 together. Assertion of VT_CANCEL alsoresults in switches 1432 and 1442 being closed to connect the twoelectrodes of capacitor 1436 together, and to connect the two electrodesof capacitor 1446 together.

In response to AMPLIFY being asserted at time 923 of RC1, and thereforeAMPLIFYB being negated (not shown), switches 1410 and 1420 are open, andswitches 1430 and 1440 are closed to force current Ie_(sense) andcurrent Iw_(sense) through transistors 1435 and 1445, respectively,during precharge. Note that current Ie_(sense) and current Iw_(sense)are typically matched currents. In response to forcing currentIe_(sense) through transistor 1435 while its gate and drain areconnected, a voltage between the gate and source of transistor 1435 willbe set at the threshold voltage of transistor 1435 (Vt₁₄₃₅), and avoltage equal to V_(EQ)+Vt₁₄₃₅ will be provided to the sense node SNw asindicated during time 916 of RC1. Similarly, a voltage between thesource and gate of transistor 1445 will be set at the threshold voltageof transistor 1445 (Vt₁₄₄₅), and a voltage equal to V_(EQ)+Vt₁₄₄₅ willbe provided to the sense node SNe.

A closed loop circuit path is formed as follows: the drain of transistor1435 is electrically connected, through closed switch 1431 to anelectrode of capacitor 1447, the other electrode of capacitor 1447 iselectrically connected to the source of transistor 1435, through closedswitches 1461 and 1462. As a result of this closed loop circuit path,capacitor 1447 is charged to threshold voltage of transistor 1435(Vt₁₄₃₅) that results between the source and drain of transistor 1435during time 916. Another closed loop circuit path is formed as follows:the drain of transistor 1445 is electrically connected to an electrodeof capacitor 1437, through switch 1441, the other electrode of capacitor1437 is electrically to the source of transistor 1445, through switches1461 and 1462. As a result of this closed loop circuit path, capacitor1437 is charged to the threshold voltage of transistor 1445. As will bedescribed further herein, the effect of storing charges at capacitors1437 and 1447 is to provide transistor 1435 and transistor 1445 withidentical effective voltage thresholds of zero volts when operating aspart of a differential amplifier.

Time 911 represents the beginning of a formal read cycle labeled RC2. Attime 911, signals Vt_CANCEL and AMPLIFY are negated. In response toVt_CANCEL being negated, switches 1431 and 1441 are opened, whereby thesource and drain of transistor 1435 are no longer electrically connectedtogether, and the source and drain of transistor 1445 are no longerelectrically connected together. In response to AMPLIFY being negated attime 911, and AMPLIFYB being asserted (not shown), switch 1410 is closedwhile switch 1430 is opened, thereby redirecting the current Iw_(sense)from through switch 1430 to GBLw, which is connected to INw, throughswitch 1410. Similarly, in response to AMPLIFY being negated switch 1420is closed while switch 1440 is opened, thereby redirecting the currentIe_(sense) from through switch 1440 to GBLe, which is connected to INe,through switch 1420.

In further response to switch 1410 and switch 1420 being closed at time911, voltages at SNw and SNe will be set to the pre-charge voltage(V_(EQ)) being applied to GBLw and GBLe. During the time from 911 to 912a closed loop circuit path is formed as follows: from the gate oftransistor 1435 to an electrode of capacitor 1447, to the otherelectrode of capacitor 1447, to the source of transistor 1435 throughclosed switches 1461, and 1462. As a result of this closed loop circuitpath the gate of transistor 1435 is biased above the voltage (V_(EQ)) atthe source of transistor 1435 by the voltage at capacitor 1447, i.e.,Vt₁₄₃₅. Another closed loop circuit path is formed as follows: from thegate of transistor 1445 to an electrode of capacitor 1437, to the otherelectrode of capacitor 1437, to the source of transistor 1445 throughclosed switches 1461, and 1462. As a result of this closed loop circuitpath the gate of transistor 1445 is biased above the voltage (V_(EQ)) atthe source of transistor 1445 by the voltage at capacitor 1437, i.e.,Vt₁₄₄₅.

At time 912 of RC2, one of the signals READw or READe is asserted.Signal READw is asserted if during the current read cycle data is beingread at bit line GBLw. Signal READe is asserted if data during thecurrent read cycle is being read at bit line GBLe. For purposes ofdiscussion it assumed that storage location 1494 is being read resultingin data information at GBLw, therefore, READw is asserted.

In response to READw being asserted, a portion of the charge stored atcapacitor 1437 is shared with capacitor 1436, thereby modifying thevoltage across capacitor 1437 by an offset amount, Voffset, that willaffect the effective Vt of transistor 1445 during a subsequent amplifyphase of the read cycle RC2. The value of Voffset is defined based uponthe size of capacitor 1436 relative to the size of capacitor 1437, whichis defined to obtain a desired Voffset. For example, assume that priorto switch 1433 being closed capacitor 1437 has a storage area of 9 areaunits and a charge of 10 charge units, and the capacitor 1437 has astorage area of 1 area unit and a charge of 0 charge units. After switch1433 is closed at time 912, 1 charge unit will be stored at thecapacitor 1436 and 9 charge units will be stored at capacitor 1437. Thisapproximate 11% drop in charge units at capacitor 1437 corresponds to anapproximate 11% drop of the voltage across transistor 1445, andtherefore an approximate 11% drop in the effective Vt of transistor1445. Signal READw is negated prior to time 913. Voffset can be in therange of approximately less than one-half a voltage threshold of thetransistor it is biasing, such as transistor 1435, and greater than thevoltage offset due to natural mismatch between adjacent devices, such astransistors 1435 and 1445 or capacitors 1437 and 1447. For example, theVoffset can be in the range of approximately 750-20 millivolts, such asless than 300 millivolts, less than 250 millivolts, less than 200millivolts, less than 150 millivolts, less than 100 millivolts, and lessthan 100 millivolts.

At time 913, signal PC_GBL is negated to electrically isolate GBLw fromGBLe. Therefore, while the bottom electrode of capacitor 1447 iselectrically connected to both INw and INe during the time period priorto the beginning of time 913, during time period 921, the bottomelectrode of capacitor 1447 is electrically connected to INe, but is notelectrically connected to INw. Note that INw represents a node of senseamplifier 14 that is electrically connected to GBLw and that INerepresents a node of sense amplifier 14 that is electrically connectedto GBLe during the read cycle RC2. The time period 921, from when PC_GBLis negated until AMPLIFY is asserted is referred to as the integrationperiod 921. During the integration period 921, transistor 1492 acts as areference transistor having a reference conductance based upon thevoltage Vref, and transistor 1491 acts as a buffer for signal Vdata andwill have a conductance, referred to as a data conductance, that isbased upon a program state of the storage cell 1494. The dataconductance will be either matched or significantly different than thereference conductance.

During integration period 921, the transistor 1492 is highly conductivedue to storage cell 1494 being non-conductive. When transistor 1492 ishighly conductive charge at SNe, e.g., charge introduced by currentIe_(sense), flows to ground, whereby a voltage at SNe transitions fromV_(EQ) toward ground at a first rate that is based upon the currentIe_(sense) and the conductance of GBLe at INe.

In response to the storage cell 1494 being read during time period RC2while in a non-conductive state, and in response to Vref being equal toV_(PWR), the voltage Vdata at the gate of transistor 1491 will be thesame as the voltage Vref at the beginning of the integration period 921.As a result, the conductances at the inputs INw and INe of the senseamplifier are the same, assuming transistors 1491 and 1492 are matchedtransistors. Therefore, INw transitions from V_(EQ) toward ground at arate that INe transitions from V_(EQ) toward ground. As a result, at theend of the integration period 921 of RC2 the data signals at INw and SNware at substantially the same voltage as the reference signal at INe andSNe. For example, the data signal voltage, e.g., the voltage at INw, isless than the sum of reference signal at INe (V_(INe)) and offsetvoltage Voffset. Therefore, when V_(INw) is less than V_(INe)+Voffset atthe end of the integration period 921, the conductance at the input INwof the sense amplifier 14 is considered to be substantially the same asthe conductance of the second input of the sense amplifier during theintegration period 921.

A time period 922 begins at time 914 when the signal AMPLIFY isasserted, and AMPLIFYB negated (not shown). During time period 922,transistors 1410 and 1420 are opened to isolate SNw from INw and SNefrom INe. During time period 922, transistors 1430 and 1440 are closedto communicate information at sense nodes SNw and SNe to the drainelectrodes of transistors 1435 and 1445, respectively.

During time period 922 of read cycle RC2, the gate and source oftransistor 1445 are biased as follows: the source of transistor 1445 isbiased at the voltage at INe, which is the voltage V_(GBLe) at GBLe, andthe gate of transistor 1445 is biased by the sum of the voltage at INw,which is the voltage V_(GBLw) at GBLw, and the voltage across capacitor1437, which is (V_(T1445)−Voffset). The gate to source voltage drop attransistor 1445 is therefore equal toV_(INw)+(V_(T1445)−Voffset)−V_(INe). Therefore, the effective Vt oftransistor 1445 with respect to INw and INe is Voffset, wherebytransistor 1445 will remain off during time period 922 of RC2 becausethe voltage V_(INe) is less than the sum of V_(INw) and Voffset. As aresult of transistor 1445 being turned off during time period 922 ofread cycle 922, V_(SNE) increases while voltage V_(INe) remains at, orcontinues its transition to, V_(GND).

During time period 922 of read cycle RC2, the gate and source oftransistor 1435 are biased as follows: the source of transistor 1435 isbiased at the voltage at INw, which is the voltage V_(GBLw) at GBLw; andthe gate of transistor 1435 is biased by the sum of the voltage at INe,which is the voltage V_(GBLe) at GBLe, and the voltage V_(T1435) acrosscapacitor 1447. The gate to source voltage drop at transistor 1435 istherefore equal to V_(INe)+V_(T1435)−V_(INw). Therefore, the effectiveVt of transistor 1435 with respect to INe and INw is zero, wherebytransistor 1435 will begin to turn on when the voltage V_(INw) is equalto or greater than V_(INe). Since V_(INw) and V_(INe) are transitioningtoward the same reference voltage, e.g., V_(GND), transistor 1435 willturn on during time period 922 of read cycle RC2, thereby maintainingV_(SNw) at a relatively low voltage as compared to voltage V_(SNe), andassuring the voltage at V_(GBLw) remains at or above the voltage atV_(GBLe).

At time 915 a latch signal LAT is asserted to latch a first logic state,illustrated as a high voltage level at FIG. 4, based upon the voltagedifference between V(SNw) and V(SNe).

At time 916, signals PC_GBL and Vt_CANCEL are asserted as previouslydescribed.

FIG. 5 illustrates the end of read cycle RC2 and the beginning of a newread cycle labeled RC3. Operation of the sense amplifier 141 is the samefrom time 912 until time 914 of read cycle RC3 as previously describedfor read cycle RC2. Therefore, transistors 1435 and 1445 are biased inthe same manner just prior to the beginning of the integration period921 of read cycle RC3 as just prior to the beginning of the integrationperiod 921 of RC2. However, during read cycle RC3, storage cell 1494 isin a conductive state resulting in Vdata having a voltage less than thevoltage of V_(REF). For example, at time 914 Vdata can be at or nearV_(GND) and V_(REF) can be at V_(PWR).

During integration period 921 of read cycle RC3, the transistor 1492 isnon-conductive due to storage cell 1494 being conductive. Whentransistor 1492 is non-conductive, charge introduced by currentIw_(sense), remains at SNw and GBLw, which causes the voltage at SNw andSNw to increase as illustrated at FIG. 5. However, the voltage at SNeand GBLe transitions from V_(EQ) toward ground as previously describedsince transistor 1492 has the same conductance as previously describedbased upon the reference signal Vref.

Therefore, at the beginning of the amplify period 922 of read cycle RC3,the voltage V_(INw) is greater than the sum of V_(INe) and Voffsetcausing transistor 1445 to be turned on and transistor 1435 being turnedoff. As a result of transistor 1445 being turned on during time period922 of read cycle RC3, V_(SNe) remains at, or continues its transitionto, V_(GND) along with V_(INe), while V_(SNw) increases as a result oftransistor 1435 being turned off during time period 922 of read cycleRC3.

At time 915 of RC3 a latch signal LAT is asserted to latch a secondlogic state, illustrated as a low voltage level at signal DATA of FIG.5, based upon the voltage difference between V(SNw) and V(SNe).

FIG. 6 illustrates a method in accordance with a specific embodiment ofthe present disclosure. At block 1011, it is determined which input of asense amplifier is a data input to receive a data signal during aspecific portion of a read cycle, such as portion 921 as previouslydiscussed, and which input of the sense amplifier is a reference inputto receive a reference signal during the first portion of the readcycle. For example, based upon specific address and control information,the decode module 18 determines which input of a sense amplifier will beconnected to a global bit line that is to receive a data signal andasserts one of signal READw or signal READe in response.

At block 1012 a first logic state is generated at an output of the senseamplifier in response to a conductance at the input of the senseamplifier receiving information based upon a storage cell being readbeing substantially equal to the conductance at the reference inputduring the specific portion of the read cycle. For example, as discussedwith reference to time portion 921 of the read cycle RC2 of FIG. 4, itwas determined that the conductance at INw of the sense amplifiermatches the conductance at INe of the sense amplifier when the voltagesat these inputs are within a defined amount. Conversely, when theconductances at the sense amplifier's inputs do not match during timeportion 921 neither will their respective voltages, and a second logicstate, complementary to the first logic state, represented by ahigh-voltage data signal is generated.

The conductances at the inputs of the sense amplifier match when theyare substantially equal. The conductances are considered substantiallyequal based upon a value defined at the sense amplifier. The definedvalue can be based upon charge stored at a passive storage element, suchas capacitor 1437 during the amplification portion 922, as previouslydiscussed. The charge stored at the capacitor that defines the value canbe based upon a threshold voltage. For example, the charge can beobtained by setting the voltage at the capacitor based upon a thresholdvoltage of a transistor of the sense amplifier, and modifying thischarge by an offset value. For example, the defined offset value can bebased upon a second passive storage element, such as capacitor 1436 aspreviously discussed.

FIG. 7 illustrates a method in accordance with a specific embodiment ofthe disclosure. Block 1016 represents events associated with a portionof a read cycle, such as the time period between time 916 at the end ofone read cycle and time 912 at the beginning of the next read cycle, asillustrated at FIG. 4 and FIG. 5. During this time period, a firstcapacitor, such as capacitor 1437, is charged based upon the thresholdvoltage of a first transistor of the sense amplifier, such as transistor1455, and a second capacitor, such as capacitor 1447, is charged basedupon the threshold voltage of a second transistor of the senseamplifier, such as transistor 1435.

Block 1017 represents events associated with a portion of a read cycle,such as the time period of FIG. 4 and FIG. 5 during which the signalREADw is asserted. During this time period the charge at the firstcapacitor is modified by a defined amount, e.g., Voffset, while thevoltage at the second capacitor is maintained.

Block 1018 represents events associated with a portion of a read cycle,such as the time period 922 illustrated at FIG. 4 and FIG. 5. Duringthis time period, a signal difference between a signal at the firsttransistor and a signal at the second transistor is amplified based uponthe first transistor having an effective threshold voltage equal tovoltage offset when the second transistor has an effective thresholdvoltage of zero volts.

Block 1019 represents events associated with a portion of a read cycle,such as the time period from time 916 to the end of a read cycle asillustrated at FIG. 4 and FIG. 5. During this time period a logic valuebased upon the amplified signal is determined.

Overall operation of a specific embodiment of memory device 10 isdescribed with reference to FIG. 8, which illustrates sense amplifier14, global bit line 131, global bit line 132, a portion of sub array121, and a portion of sub array 122. The illustrated portions of subarray 121 and sub array 122 are associated with NAND string module 1311and NAND string module 1321, respectively, as are further describedherein. The illustrated portion of sub array 122 has the same elementsas the illustrated portion of sub array 121. Therefore, the samereference numbers are duplicated at FIG. 8 to identify analogouselements within the portion of sub array 121 and the portion of subarray 122.

The illustrated portion of sub array 121 includes a multiplexer 271having a data output labeled OUT, data inputs labeled I0-I3, andunlabeled control inputs connected to interconnects LBSel[0 . . . 3]that provide control signals. Based upon the description herein, it willbe appreciated that multiplexer 271 acts as a switch in that it cancommunicate signal information in either direction, i.e., from an inputto output OUT during a read operation, or from output OUT to an inputduring a write operation. However, the operation of the memory device 10is primarily described herein with respect reading information at astorage cell, during which the switch operates as a multiplexer,therefore, the terms “input” and “output” are used with respect tomultiplexer 271 for convenience to correspond to operation of amultiplexer during a read operation.

Each input of multiplexer 271 is respectively connected to one of acorresponding plurality of local bit lines, i.e., inputs I0-I3 areconnected to local bit lines LBL0-LBL3, respectively.

Each local bit line of bit lines LBL0-LBL3 is respectively connected toa corresponding plurality of NAND strings. However, FIG. 8 illustrateseach respective local bit line respectively connected to only one of itscorresponding plurality of NAND strings. For example, local bit lineLBL0 is shown as connected to NAND string NS00, while other NANDstrings, such as NAND string NS01, NS02, etc., that are connected tolocal bit line LBL0 are not illustrated. Each NAND string includes adrain side select gate, a string of NAND type storage cells, and asource side select gate. The drain side select gate of a NAND stringincludes a control gate connected to a corresponding interconnectlabeled DSG[0], a drain electrode at the output of the NAND string, anda source electrode that is connected to the drain electrode of thestorage cell at the top of the NAND string. Note that the drainelectrode of the drain side select gate is labeled BL to indicate thisnode is the bit line of a particular NAND string. The source side selectgate of the NAND string includes a control gate connected to aninterconnect labeled SSG[0], a drain electrode that is connected to thesource of the storage cell that is at the bottom of the NAND string, anda source electrode. The source electrode of the source side select gateof each NAND string is connected to a reference node labeled VGND thatprovides a reference signal, such as ground, during a read cycle. Theelectrode that is common to the source electrode of the drain sideselect gate and to the drain electrode of the storage cell at the top ofthe NAND string is referred to herein as the “bit line of the NANDstring.”

Each one of the local bit lines LBL0-LBL3 of multiplexer 271 isrespectively connected to a corresponding deselect bias transistor oftransistors 291-294. Each one of the deselect bias transistors 291-294respectively includes a control gate connected to a correspondinginterconnect of interconnects LBSelB[0 . . . 3], a drain connected to acorresponding local bit line, and a source connected to referenceinterconnect VGND.

Referring to the local bit lines LBL0-LBL3 of sub array 121: local bitline LBL0 is connected to input I0 of multiplexer 271, to the output ofNAND string NS00, and to the drain of transistor 291; local bit lineLBL1 is connected to input I1 of multiplexer 271, to the output of NANDstring NS10, and to the drain of transistor 292; local bit line LBL2 isconnected to input 12 of multiplexer 272, to the output of NAND stringNS20, and to the drain of transistor 293; local bit line LBL3 isconnected to input 13 of multiplexer 271, to the output of NAND stringNS30, and to the drain of transistor 294.

A transistor 2721 of sub array portion 121 includes a gate connected toan interconnect labeled SELws, a source connected to the output ofmultiplexer 271, and a drain.

Control devices associated with the illustrated portion of sub array 121include transistor 212, buffer 91, transistor 213, and resistive element214. Transistor 212 includes a gate connected to an interconnect labeledRD, a drain forming an output that connects the illustrated portion ofsub array 121 to GBLw, and a source. Transistor 213 includes a gateconnected to an interconnect labeled WR, a drain forming an outputconnected to GBLw, and a source connected to the drain of transistor2721. A buffer 91 includes an input connected to the drain of transistor2721, and an output connected to the source of transistor 212. Theresistive element 214 includes a first electrode connected to aninterconnect labeled V_(PWR), and a second electrode connected to thedrain of transistor 2721.

A plurality of data paths that include the buffer 91 are formed at theNAND string module 121. For example, one data path includes: the node ofthe NAND string module that is connected to GBLw, buffer 91, transistor2721, multiplexer 271, and NAND string NS00. Each one of the other NANDstrings of NAND string module 212 illustrated at FIG. 8 is part ofanother data path. For example, another data path includes: the node ofthe NAND string module that is connected to GBLw, buffer 91, transistor2721, multiplexer 271, and NAND string NS10. For each such data path,the buffer 91 is connected in series between the corresponding NANDstring of the data path and the node of the NAND string module that isconnected to GBLw.

As previously indicated, the portion of sub array 122 illustrated atFIG. 8 has the same elements as the illustrated portion of sub array121. Furthermore, the elements of sub array 122 are connected to thesame external interconnects as the elements of sub array 121, except fortransistor 2721 of sub array 122 which has its gate electrode connectedto interconnect SELes, as opposed to interconnect SELws.

The portion of memory device 10 illustrated at FIG. 8 operates asdescribed below during a read operation of the storage cell of NANDstring NS00 of sub array 121 that is connected to interconnect WL[1].

At the beginning of a read cycle, the data node at the input of buffer91 has been biased to the reference voltage at reference node V_(PWR)through resistive element 214.

In response to address and control signals being received at the memorydevice 10 that indicate a the storage cell connected to interconnectWL[1] of NAND string NS00 is to be read, the following occurs: signalsat interconnects DSG[0], SSG[0], WL[1], LBSel[0], LBSelB[1 . . . 3],SELw, and RD are asserted; and signals at interconnects WL[0, 2 . . .y], LBSel[1 . . . 3], LBSelB[0], SELe, and WR are negated.

Assertion of signals at DSG[0] and SSG[0] turns on the drain sidetransistor and the source side transistor of each of the NAND stringsillustrated at FIG. 8. As a result, the bit line BL of NAND string NS00is conductively coupled to its corresponding local bit line LBL0,allowing signal information at BL to be communicated between the bitline of NAND string NS00 and its corresponding local bit line, LBL0.Also, the source of the bottom storage cell of NS00 is conductivelycoupled to the reference interconnect V_(GND).

Assertion of a signal at WL[1] provides a bias signal that allows thestorage cells connected to WL[1] to be either conductive ornon-conductive based upon their respective program state. Negation ofsignals at WL[0, 2 . . . y] during a read cycle provides a bias signalthat turns on each of the other storage cells of the NAND stringallowing the conductivity state of the storage cell being read to becommunicated to the bit line BL of the NAND string.

Assertion of the a signal at interconnect LBSel[0] allows communicationof signal information between input I0 and the output OUT of multiplexer271, while negation of the signals at interconnects LBSel[1 . . . 3]prevents communication of signal information between inputs I1-I3 andthe output OUT of multiplexer 271.

Negation of a signal at interconnect LBSelB[O] turns off transistor 291,thereby electrically isolating the local bit line LBL0 from a referencesignal at reference interconnect V_(GND). Assertion of signals atinterconnects LBSelB[1 . . . 3] turns on transistors 292-294, therebybiasing the inputs I1-I3 to the reference signal of reference nodeV_(GND) to prevent unnecessary leakage current through NAND stringsNS10, NS20, and NS30 which are not being read.

Assertion of a signal at interconnect SELws results in the output ofmultiplexer 271 being communicatively coupled to the data node at theinput of buffer 91, thereby allowing signal information at the outputOUT, which is based upon the storage cell being read, to be communicatedto the data node at the input of buffer 91. Negation of a signal atinterconnect SELes results in the output of multiplexer 271 of portion122 being electrically isolated from the data node at the input ofbuffer 91, thereby preventing signal information at the output OUT ofmultiplexer 271 of portion 122 from be communicated to its associateddata node at the input of buffer 91.

An asserted signal at interconnect RD during the read cycle results inthe output of buffer 91 being communicatively coupled to the outputnode, which is electrically connected to the global bit line GBLw tocommunicate information at the output of the buffer 91 to GBLw.

Different scenarios occur at the illustrated control portion during aread operation based upon whether a storage cell is in a conductive ornon-conductive state. In one scenario, when the storage cell being readis in a conductive state, current flowing through the storage cellresults in charge at the data node at the input of buffer 91 beingremoved. For example, current at data node 219 can flow through thetransistor 2721, the multiplexer 271, and the selected storage cell ofNS00 to the interconnect V_(GND). As a result of this current flow, thedata node at the input of buffer 91 is biased to a low voltage signal,such as near ground. The low voltage signal at the data node at buffer91 is communicated to the output of the buffer 91. In another scenario,when the storage cell being read is in a non-conductive state, nocurrent can flow through the storage cell of NS00 resulting in a chargeat the data node at the input of buffer 91 being maintained. Forexample, the voltage V_(PWR) at the data node at the input of buffer 91is maintained during the read operation. The high voltage signal at thedata node at the input of buffer 91 is communicated to the output of thebuffer 91.

In one embodiment, the buffer 91 is an open drain buffer, whereby a lowvoltage signal at its input results in the output of buffer 91 beingplaced at a high impedance state that prevents current flow to or fromthe output of portion 121, and whereby a high voltage signal at itsinput results in the output of buffer 91 being placed in a low impedancestate allowing current to flow from its output to ground. Therefore, theoutput of portion 121 can be either at a high impedance state or a lowimpedance state based upon the program state of the storage cell beingread. Note that when data is being read from portion 121 the output ofportion 122 is at a low impedance state since its associated buffer 91has a high voltage value maintained at its input.

A read signal can be asserted at interconnect RD while GBLw and GBLe arebeing precharged as previously discussed. During precharge, GBLw andGBLe are maintained at the precharge voltage. After precharge, theasserted read signal is maintained to allow the state information at theoutput of buffer 91 of portion 121 to effect the voltage at GBLw, and toallow the state information at the output of buffer 91 of portion 122 toeffect the voltage at GBLe. As previously discussed, if the output ofbuffer 91 of portion 121 is in a low impedance state, charge will beremoved from GBLw at a specific rate after precharge is completed ascurrent flows to ground through buffer 91. This rate is the same rate ascharge is removed from GBLe of portion 122 during the same period oftime. Conversely, if the output of buffer 91 of portion 121 is in a highimpedance state, charge at GBLw will build at after precharge, whileduring the same time charge continues to be removed from GBLe. Thesimilarity or difference in conductivity at the inputs of senseamplifier 14 can be detected as previously described.

FIGS. 9-31 describe specific embodiments of memory device 10 inaccordance with the present disclosures. FIG. 9 is a block diagram thatillustrates each sub array 121-126 respectively including acorresponding plurality of NAND string modules connected to itscorresponding global bit line. For example: sub array 121 includes NANDstring modules 1311-1313 connected to global bit line 131; sub array 122includes NAND string modules 1321-1323 connected to global bit line 132;sub array 123 includes NAND string modules 1331-1333 connected to globalbit line 1331; sub array 124 includes NAND string modules 1341-1343connected to global bit line 134; sub array 125 includes NAND stringmodules 1351-1353 connected to global bit line 135; sub array 126includes NAND string modules 1361-1363 connected to global bit line 136.

During operation, NAND string modules operate as pairs, each pairreferred to as a pair of NAND string modules, to provide a data signaland a reference signal to a common sense amplifier. In one embodiment,the signals provided by the NAND string modules are characterized asbeing in a high-impedance state or a low impedance state that affects acharge at their global bit lines in a manner that can be detected bytheir corresponding sense amplifiers. With respect to the NAND stringmodules of FIG. 9, a pair of NAND string modules is formed by two NANDstring modules connected to the same sense amplifier that have the sameunits digit. For example, pairs of NAND string modules associated withsub array 121 and sub array 122 include: NAND string modules 1311 and1321; NAND string modules 1312 and 1322, and NAND string modules 1313and 1323. For purposes of discussion, FIGS. 10-31 reference elementsassociated with sense amplifier 14 and the sub arrays connected to senseamplifier 14, i.e., sub arrays 121 and 122. It will be appreciated thatthe discussions related to sense amplifier 14 and its connected subarrays also pertain to the other sense amplifiers and sub arraysconnected thereto.

FIG. 10 is a block diagram illustrating a representative pair of NANDstring modules that includes NAND string module 1311 of sub array 121and NAND string module 1321 of sub array 122, that are connected toglobal bit line 131 and 132, respectively. With reference to senseamplifier 14, global bit line 131 is also referred to as GBLw and globalbit line 132 is also referred to as GBLe. It will be appreciated thatthe discussions related to the pair of NAND string modules of FIG. 10also pertain to the other pairs of NAND string modules of sub array 121and sub array 122.

NAND string module 1311 is illustrated to include a deselect bias module24, a NAND string block 23, a select module 22, a local amplifier 21, adeselect bias module 29, a NAND string block 28, and a select module 27.

Deselect bias module 24 is connected to decode module 18 viainterconnects labeled LBSelB[0 . . . 3]. NAND string block 23 isconnected to decode module 18 via interconnects DSG[0 . . . x],interconnects WL[0 . . . y], and interconnects SSG[0 . . . x], where “x”is an integer indicating the number of NAND string modules in each subarray 121-126 and “y” is the number of storage cells associated witheach NAND string. Select module 22 is connected to decode module 18 viainterconnect SELwn, and interconnects LBSel[0 . . . 3]. A plurality ofinterconnects labeled LBL0 n-LBL3 n connect the NAND string block 23 tothe deselect bias module 24 and to the select module 22. An interconnectlabeled LBLn connects select module 22 to the control module 21.

Deselect bias module 29 is connected to decode module 18 (FIG. 1) viainterconnects labeled LBSelB[0 . . . 3]. NAND string block 28 isconnected to decode module 18 via interconnects DSG[0 . . . x],interconnects WL[0 . . . y], and interconnects SSG[0 . . . x]. Selectmodule 27 is connected to decode module 18 via interconnect SELws, andinterconnects LBSel[0 . . . 3]. A plurality of interconnects labeledLBL0 s-LBL3 s connect the NAND string block 28 to the deselect biasmodule 29 and to the select module 27. An interconnect labeled LBLsconnects select module 27 to the control module 21. In a particularlayout, the control module 21, which includes buffer 91 described above,physically resides between the NAND strings 23 and the NAND strings 28.

NAND string module 1321 is similar to NAND string module 1311. Elementsof NAND string module 1321 having the same reference number as anelement of NAND string module 1311 can be implemented in the same mannerand have the same function. In addition, the elements of the NAND stringmodule 1321 are connected to the same control interconnects from decodemodule 18 as the elements from NAND string module 1311, with theexception of the select modules 22 and 27 of NAND string module 1321.Select module 22 of NAND string module 1321 is connected to aninterconnect labeled SELen as opposed to select module 22 of NAND stringmodule 1311 which is connected to an interconnect labeled SELwn. Selectmodule 27 of NAND string module 1321 is connected to an interconnectlabeled SELes as opposed to select module 22 of NAND string module 1311which is connected to an interconnect labeled SELws. In the embodimentdescribed herein only one of interconnect SELwn, SELen, SELws, and SELwnwill receive an asserted signal during a specific read cycle.

Each NAND string block of a NAND string module respectively includes aplurality of NAND string pairs. For example, FIG. 11 is a block diagramillustrating NAND string block 28, which includes NAND strings pairs31-33. Each NAND string pair includes a corresponding two sets of NANDstrings that can be mirrored so that their respective source side selectgates share a common portion of an active region as described in greaterdetail herein. Each NAND string pair is connected to two interconnectsof the interconnects DSG[0 . . . x], two interconnects of theinterconnects SSG[0 . . . x], to the interconnects WL[0 . . . y], and tothe interconnects LBL0 s-LBL3 s. For example, NAND string pair 31 isconnected to interconnects DSG[0,1], interconnects WL[0 . . . y], andinterconnects SSG[0,1]. NAND string pair 32 is connected tointerconnects DSG[2,3], interconnects WL[0 . . . y], and interconnectsSSG[2,3]. NAND string pair 31 is connected to interconnects DSG[x-1,x],interconnects WL[0 . . . y], and interconnects SSG[x-l,x].

Referring to FIG. 12, NAND string pair 32 of FIG. 11 is illustrated ingreater detail to include a set of NAND strings 321 and a set of NANDstrings 322. Each set of NAND strings 321 and 322 is illustrated to haveoutputs labeled LBL0-LBL3 and a plurality of NAND strings labeledNS0-NS3 formed at adjacent columns of sub array 121. For example, thesets of NAND strings 321 and 322 of NAND string pair 32 respectivelyinclude a corresponding set of four individual NAND strings, labeledNS0-NS3, with an output of each NAND string of the set of NAND stringsrespectively connected (not illustrated at FIG. 12) to a correspondingoutput of NAND string pair 32. Referring further to FIG. 12: the outputof NAND string NS0 of the set of NAND strings 321 and the output of NANDstring NS0 of the set of NAND strings 322 are both connected tointerconnect LBL0; NAND string NS1 of the set of NAND strings 321 andNAND string NS1 of the set of NAND strings 322 are both connected tointerconnect LBL1; NAND string NS2 of the set of NAND strings 321 andNAND string NS2 of the set of NAND strings 322 are both connected tointerconnect LBL2; NAND string NS3 of the set of NAND strings 321 andNAND string NS3 of the set of NAND strings 322 are both connected tointerconnect LBL3. Each of the sets of NAND strings 321 and 322 areconnected to a reference node 329, labeled V_(GND), which can be aground reference. It will be appreciated that the discussions related toNAND string pair 32 also describe the other NAND string pairs of theNAND string block 28 (FIG. 10).

FIG. 13 illustrates the connections of NAND string pair module 32 ingreater detail. The NAND string pair module 32 is formed within afootprint defined by a plurality of memory array columns. Eachinterconnect of interconnects WL[0 . . . y] is respectively connected toa corresponding input WL0-WLy of the NAND string pair module 32. Theseinputs respectively correspond to the inputs of the set of NAND strings321 and the set of NAND strings 322 having the same names. Theinterconnect DSG[0] (not shown) is connected to input DSGt of the NANDstring pair module 32, which is connected to the input DSG of NANDstring 321. The interconnect DSG[1] (not shown) is connected to inputDSGb of the NAND string pair module 32, which is connected to the inputDSG of NAND string 322. The interconnect SSG[0] (not shown) is connectedto input SSGt of the NAND string pair module 32, which is connected tothe input SSG of NAND string 321. The interconnect SSG[1] (not shown) isconnected to input SSGb of the NAND string pair module 32, which isconnected to the input SSG of NAND string 322. The source electrodes ofeach transistor connected to SSGt and SSGb are connected via a node 329.

FIG. 14 illustrates portions of NAND string module 1311 in greaterdetail, including select module 22, select module 27, and control module21. As previously discussed with reference to FIG. 8, control module 21includes transistor 212, transistor 213, and resistive element 214.Buffer 91 of FIG. 8 is represented as transistor 211 having a controlgate corresponding to the input of Buffer 91, a drain electrodecorresponding to the output of buffer 91, and a source connected toreference interconnect V_(GND). Buffer 91, therefore, operates as anopen drain buffer as previously described. The data node 219 receivesconductivity information from a storage cell through either selectmodule 21 or select module 27 when data is being access from NAND stringmodule 1311. For example, a signal is asserted at interconnect SELwnwhen data is to be read from a storage cell associated with selectmodule 22, and a signal is asserted at interconnect SELws when data isto be read from a storage cell associated with select module 27.

FIG. 15 represents a circuit diagram corresponding to portions of a pairof NAND string modules 121 and 122. For purposes of discussion, the pairof NAND string modules is assumed to be NAND string module 1311 of subarray 121 (see FIG. 9) and NAND string module 1321 of sub array 122 aspreviously discussed.

The circuit of FIG. 15 has been formed at memory array columns 151-154that correspond to NAND string module 1311 of sub array 121, and atmemory array columns 155-158 that correspond to NAND string module 1321of sub array 122. Horizontal lines of FIG. 15 that intersect transistorsgates represent gate electrodes. Horizontal lines that lie betweentransistor gate locations represent conductive interconnects, such asmetal lines at a specific metal layer. Vertical lines that run throughillustrated transistors represent conductive interconnects, such asmetal lines at specific metal layers, that overly the transistors. Forexample, the solid line from the source of the transistor at column 151and row 171 to the drain of the transistor at column 151 and line 176represents a metal line and associated contacts. A solid dot at the gateof a transistor indicates that the transistor is used to implement aspecific function associated with the NAND string module based upon asignal provided to its control gate. Transistors associated with a soliddot are illustrated using solid lines. An open dot at the gate of atransistor indicates a transistor that receives an isolating controlsignal, such as ground, to prevent current flow across the transistor'schannel. Transistors associated with an open dot are illustrated usingdashed lines that appear lighter than transistors having a solid dot attheir gate. No dot at the gate of a transistor indicates that thetransistor is not used to implement a function associated with the NANDstring module. These transistors are typically bypassed using anoverlying metal interconnect. In the discussion below, a specifictransistor is referenced using its column and row number. For example,the transistor at column 154 and row 173 can be referred to herein astransistor 154/173.

With respect to NAND string module 1311, control portion 21 (FIG. 10) isformed from transistors at rows 163-168 of columns 151-154. Transistorsat rows 162 and 169 are connected to an interconnect that receives anegated signal during data access operations to prevent current flowthrough their channel regions, thereby electrically isolating portionsof the active region within a column from one another. Elements ofcontrol portion 21 correspond to elements of FIG. 14 as follows:transistor 211 corresponds to transistor 152/168 and transistor 153/168,which are connected in parallel; transistor 212, corresponds totransistor 152/167 and transistor 153/167, which are connected inparallel; transistor 213 corresponds to transistor 152/166; resistiveelement 214 is formed by a block of nine transistors connected inseries, where the transistors are common to columns 152-154 and rows163-165.

With respect to NAND string module 1311, select module 27 of FIG. 14 isformed from transistors at rows 171-176 of columns 151-154. Thetransistor 2721 corresponds to four transistors: transistor 151/171,transistor 152/171, transistor 153/171, and transistors 154/171. Themultiplexer 271 corresponds to four transistors: transistor 151/176,transistor 152/175, transistor 153/174, and transistor 152/173.

With respect to NAND string module 1311, the NAND string block 28 asillustrated at FIG. 10, is formed from transistors at rows 177-181 ofcolumns 151-154. For example, NAND string NS0 corresponds to thefollowing transistors: transistor 151/177 is the drain side selecttransistor; transistors 151/178-180 are storage cells of the NANDstring; and transistor 151/181 is the source side select transistor. Itwill be appreciated that additional NAND strings that are notillustrated at FIG. 15 are formed at column portions between row 181 androw 182.

With respect to NAND string module 1311, the de-select bias module 29,as illustrated at FIG. 10, is formed from transistors at rows 182-185 ofcolumns 151-154. Referring to FIG. 8, transistor 291 corresponds totransistor 151/185. Transistor 292 corresponds to transistor 152/184.Transistor 293 corresponds to transistor 153/183. Transistor 294corresponds to transistor 154/182.

The transistors of columns 155-158 are associated with NAND stringmodule 1321, and are connected in the same manner as discussed withrespect to the transistors of columns 151-154 of NAND string module1311, except that the transistor 2721 of NAND string module 1321 (FIG.8) corresponds to transistor 155/172, transistor 156/172, transistor157/172, and transistor 158/172.

It will be appreciated that FIG. 15 only illustrates the control portion21 of FIG. 8 and those portions below control portion 21 of NAND stringmodule 1311 as illustrated at FIG. 8, and that portions of NAND stringmodule 1311 above control portion 21 can be implemented by mirroring thecircuit at columns 151-154, whereby the mirrored circuit would beconnected to the data node 219.

FIGS. 16-20 illustrate in physical stick layout form key features of aprocess flow used to form the elements of the NAND string modules thatmake up the memory array 12 from transistors formed at the active regionof various columns. The column and row numbers of FIG. 15 have beenmaintained to correlate transistors of FIG. 15 to correspondingtransistor locations of FIGS. 16-20. Therefore, transistors at column151 of FIG. 15 include a portion of the active region of column 151 asillustrated at FIGS. 16-20.

FIG. 16 illustrates active regions formed at columns 151-154 of memoryarray 12, whereby sequentially numbered columns are immediately adjacentto each other. The pitch of each of the columns 151-154 can be the same,or vary periodically. Similarly, the pitch of columns 155-158 can be thesame or vary periodically. Each column of the memory array 12 has alength, though FIG. 16 illustrates only a portion of the length of acolumn that includes those portions of the NAND string module 1311illustrated at FIG. 15. Each column of the columns 151-154 respectivelyincludes a corresponding active region that extends the length of thesub array 121. Each NAND string module connected to the correspondingglobal bit line 131 of sub array 121, including NAND string module 1311,is formed at the columns 151-154. With respect to an active region, froma top view, the active region has a length dimension extending in onedirection and a width dimension extending in a another dimension,wherein the width dimension is the same as or less than the lengthdimension and is orthogonal to an edge of the active region. In oneembodiment a width of the active region can be approximately 45 nm andthe column pitch of the active region can be approximately 90 nm. FIG.24 illustrates a cross sectional view along a column length dimension ofFIG. 16 at a location that includes the active region 1540 of column 154that comprises a P-doped material 1541. FIG. 25 illustrates a crosssectional view along a column width dimension of FIG. 16 at a locationbetween rows 165 and 166 that illustrates active regions portions 1511,1521, 1531, and 1541 isolated from each other by isolation regions 1512,1522, 1532, and 1542 of columns 151-154.

At FIG. 17, gate structures have been formed overlying the activeregions where transistors are to be formed at each of the rows 161-185.It will be appreciated that not all gate structures illustrated at FIG.17 need to be formed using the same process, even though the transistorsbeing formed by the illustrated gate structures, whether storage cellsor switches, are formed at different portions of the same columns. Forexample, the transistors at rows 178-180 that form storage cells can beformed can be formed using a different process than the transistors thatperform a switching function, such as the transistors at row 177. Thegate structures associated with control functions are illustrated to bewider than gate structures associated with storage cells and resistiveelement 214. FIG. 26 illustrates the cross sectional view at the samelocation as FIG. 24 after formation of the gate structures, which areillustrated to include a gate dielectric between a channel region and aconductive portion.

At FIG. 18, respective conductive regions have been formed just belowcorresponding rows 161, 169, 181, and 185 to electrically connectportions of active regions together. For example, the conductive regionscan be metal interconnects, such as an interconnect including aluminum,copper, tungsten, or the like. FIG. 27 illustrates the cross sectionalview at the same location as FIG. 24 after formation of the metalinterconnects.

At FIG. 19, conductive contacts and interconnect regions, e.g., metallines, have been formed overlying the conductive gate layer to connecttransistors as described at FIG. 15. Vertical portions of conductiveinterconnect regions formed at FIG. 19 are illustrated as being offsetfrom the active regions formed at FIG. 16 for purposes of illustration.Typically, the vertical portions of conductive interconnect regionsformed at FIG. 19 will directly overly the active regions. Theconductive interconnect regions can be any conductive material suitablefor the purposes described herein. For example, the conductive regionsformed at FIG. 19 can be metal interconnects, such as an interconnectincluding aluminum, copper, tungsten, or the like. A contact illustratedat FIG. 19 connects together those features that it abuts. For example,the contact between row 161 and row 162 connects a horizontal conductiveline between row 161 and row 162 to the conductive interconnect ofcolumn 154 that overlies the gate structure at row 162. Note that thiscontact also connects the underlying active region shared by transistors154/161 and 154/162, even though this connection to the shared activeregion has no operational effect in that the neighboring transistors arenot active during normal operation. The contact at column 154 betweenrows 162 and 163 connects the active region shared by transistors154/162 and 154/163 to the conductive interconnect of column 154 thatoverlies the gate structure of row 162. The contact abutting the gateelectrode at row 168 connects the gate structure of row 168 to theoverlying vertical conductive interconnect formed at FIG. 19. Note thatthis contact also connects the underlying active region shared bytransistors 151/167 and 151/169, even though this connection has nooperational effect. Note that in an alternate embodiment, the gate atrow 168 can extend over the active region of column 151, thereby forminga transistor, with no adverse effects so long as the gate at row 168 andthe overlying metal line are electrically connected by the contact.

FIG. 28 illustrates the cross sectional view at the same location asFIG. 24 after formation of the metal interconnects and contacts, andFIG. 29 illustrates the cross sectional view at the same location asFIG. 25 after formation of the metal interconnects and contacts.

FIG. 20 illustrates the NAND string module features of columns 151-158,as described with reference to the NAND string modules 1311 and 1321 ofFIG. 15, in stick layout form.

FIG. 21 illustrates a method in accordance with a specific embodiment ofthe present disclosure. At block 1021, a first NAND string of a firstplurality of NAND strings of a NAND string module is selected. Forexample, in the specific embodiments previously discussed, any one ofthe NAND strings associated with NAND string module 1311 can be selectedby the decode module 18 during a read operation.

At block 1022, information at a bit line of the first NAND string iscommunicated to the output of the first NAND string module through abuffer in response to the first NAND string being selected. For example,as illustrated at FIG. 8, information at a bit line, BL, of NAND stringNS00 can be communicated to global bit line 131, GBLw, through buffer 91of the NAND string module 1311 in response to a storage cell of NANDstring NS00 being selected during a read operation.

At block 1023, a second NAND string of the first plurality of NANDstrings of the NAND string module is selected. For example, in thespecific embodiment previously discussed, any other NAND stringassociated with the NAND string module 1311 can be selected by thedecode module 18 during a read or write operation. The first and secondNAND string can be in the same NAND string block, i.e., both NAND stringblock 28, or in different NAND string blocks, i.e., one in NAND stringblock 23 and the other in NAND string block 28.

At block 1024, information at a bit line of the second NAND string iscommunicated to the output of the first NAND string module through thebuffer in response to being selected. For example, a specific embodimentpreviously described indicates that information from each NAND string ofa NAND string module is communicated to the output of the NAND stringmodule through the same buffer for all read operations. It will beappreciated that information from all NAND strings associated with NANDstring module 1311 is communicated to the output of the NAND stringmodule through buffer 91.

At block 1025, a third NAND string of the second plurality of NANDstrings of the NAND string module is selected. For example, in thespecific embodiment previously discussed, the first NAND string and thesecond NAND string are associated with NAND string block 28 and thethird NAND string is associated with NAND string block 23.

At block 1026, information at a bit line of the third NAND string iscommunicated to the output of the first NAND string module through thebuffer in response to being selected. For example, a specific embodimentpreviously described indicates that information from each NAND string ofa NAND string module is communicated to the output of the NAND stringmodule through the same buffer for all read operations.

FIG. 22 illustrates a method in accordance with a specific embodiment ofthe present disclosure. Block 1031 corresponds to block 1021 of FIG. 21as previously discussed subject to other limitations of the method ofFIG. 22.

At block 1032, a bit line of the first NAND string is electricallyconnected to the output of the first NAND string module through a bufferand a first multiplexer in response to being selected. For example, asillustrated at FIG. 8, a bit line BL of a NAND string module at subarray 121 can be selected to communicate information to global bit line131 through buffer 91 and through multiplexer 271 to the output of theNAND string module in response to being selected during a readoperation.

Block 1033 corresponds to block 1023 of FIG. 21 as previously discussedsubject to other limitations of the method of FIG. 22.

At block 1034, information at a bit line of the second NAND string iscommunicated to the output of the first NAND string module through thebuffer and the first multiplexer in response to being selected. Forexample, information at two NAND strings associated with the same NANDstring block, such as NAND string block 28, will be communicated to theoutput of the NAND string module through the same buffer andmultiplexer, such as local bit line select module 271, for all readoperations.

At block 1035, a third NAND string of a second plurality of NAND stringsof the NAND string module is selected subject to other limitations ofthe method of FIG. 22.

At block 1036, information at a bit line of the third NAND string iscommunicated to the output of the first NAND string module through thebuffer and a second multiplexer in response to being selected. Forexample, information at two NAND strings associated with the twodifferent NAND string blocks, such as NAND string block 23 and NANDstring block 28, will be communicated to the output of the NAND stringmodule through the same buffer and different multiplexers during theirrespective read operations.

FIG. 23 illustrates a flow diagram of a method according to a specificembodiment of the present disclosure. At block 1041, a first NAND stringof a first plurality of NAND strings is selected. For example, in thespecific embodiment previously discussed, any one of the NAND stringsassociated with NAND string module 1311 can be selected by the decodemodule 18 during a read or write operation.

At block 1042, information at a bit line of the first NAND string iscommunicated to the output of the first NAND string module, which iselectrically connected to a first global bit line, e.g., GBLw, through abuffer in response to being selected during a read operation. Forexample, as illustrated at FIG. 8, a bit line BL of a NAND string, suchas NAND string NS00, can be selected to communicate information toglobal bit line 131 through buffer 91 to the output node of the NANDstring module in response to being selected during a read access of aNAND storage cell of the first NAND string.

At block 1043, a second global bit line, e.g., GBLe, is biased basedupon a reference signal, e.g., Vref, in response to the read operationof the first NAND string.

At block 1044, a first logic value is provided to an output node, suchas to the output node DATA of a sense amplifier, in response to biasingthe second global bit line when a storage cell being read is at anon-conductive state. For example, when the second global bit line isbiased based upon Vref when a storage cell being read is non-conductive,the sense amplifier will apply a first logic value.

At block 1045, a second logic value is provided to an output node, suchas to the output node DATA of a sense amplifier, in response to biasingthe second global bit line when a storage cell being read is in aconductive state. For example, when the second global bit line is biasedbased upon Vref when a storage cell being read is conductive, the senseamplifier will apply a second logic value.

In the foregoing specification, various benefits and advantages havebeen described with reference to specific embodiments. However, one ofordinary skill in the art would appreciate that various modificationsand changes can be made without departing from the scope of the presentinvention, as set forth in the claims. Accordingly, the specificationand figures are to be regarded in an illustrative rather than arestrictive sense, and all such modifications are intended to beincluded within the scope of the present disclosure.

For example, each of the sense amplifiers described above have beenassociated with a single pair of inputs. It will be appreciated however,that each sense amplifier can be associated with 2 or more pairs ofsense amplifiers. For example, FIG. 30 illustrates an alternateembodiment of a sense amplifier that is connected to two pairs of globalbit lines. Amplifier 1411 and precharge module 1412 of FIG. 30 are thesame as previously described at FIG. 3. A module 1413 selects one on twopairs of global bit lines to be sensed during a read cycle. A signal isasserted at interconnect SEL_wPGBL is asserted to select global bitlines GBLw1 and GBLe1. A signal is asserted at interconnect SEL_e isasserted to select global bit lines GBLw2 and GBLe2.

A specific implementation of Latch 143 is illustrated at FIG. 30. Duringa write mode data is received at interconnect DINw that is to be writtento a storage cell associated with the global bit line connected to inputINw, and data is received at interconnect DINe that is to be written toa storage cell associated with the global bit line connected to inputINe. During a read mode data read from a storage cell associated withthe global bit line connected to input INw is provided to interconnectDINw, and data read from a storage cell associated with the global bitline connected to input INe is provided to interconnect DINe. The signalHOLDB is asserted to provide power to the latching transistors. SignalWORD_ADDR connects the output of the latch to an output data bus.

A specific implementation of the current sources 144 and 145 areillustrated as being formed from PFETs. The transistors used to form thePFETS of current sources 144 and 145 can be interleaved in a layout toensure matching.

FIG. 31 illustrates a specific layout implementation of pairs of globalbit lines to reduce the affects of cross talk between adjacent cells,whereby adjacent pairs of global bit lines are twisted with respect toeach other at an interval equal to one-quarter of the length of a globalbit line. For example, the global bit lines associated with senseamplifiers SA1 and SA3 twist at a location close to the senseamplifiers, labeled 0/4, and at a location near the center of the globalbit lines, labeled 2/4. The global bit lines associated with senseamplifiers SA2 and SA4 twist at a location about one-quarter of thelength global bit lines from the sense amplifiers, labeled 1/4, and at alocation about three-quarters of the length of the global bit lines,labeled 3/4. Twisting the global bit lines in this manner results in anysignal-cross coupling noise being distributed equally to both global bitlines in a global bit line pair such that the noise can be effectivelyremoved by common mode rejection at the differential sense amplifiers.It will be appreciated that additional twists can be introduced betweenthe quartiles illustrated at FIG. 31 to further ensure cancellation ofnoise.

It will be appreciated that other architectural features of the memorydevice 10 can be implemented than those specifically described above.For example, referring to FIG. 1 the sense amplifiers 17 can be formednear the center of an integrated circuit between two memory arrays. Adifferent number than the illustrated of columns can be associated witheach NAND string module. For example, more or less than four columns canbe associated with each NAND string module. When one column isassociated with a NAND string module, the local bit line select logic271 and 221 is not needed. Alternatively, the number of columnsassociated with a specific NAND string module can be selected based uponthe pitch of the sense amplifiers. For example, the number of columnsassociated with a pair of NAND string module can be selected to be equalto the pitch of the sense amplifiers.

The NAND string module illustrated herein uses an open drain buffer tosupport a sense amplifier that detects the same conductance at itsinputs. In an alternate embodiment, the buffer can be a push-pull bufferformed within the columns to support a sense amplifier that detectsdifferential signal at its inputs. The NAND string module illustratedherein includes select transistors, such as transistors 2221 and 2721 ofFIG. 14. In alternate embodiments, the transistors 2221 and 2721 can beomitted and only the multiplexers 221 and 271 used to select betweenNAND string modules.

Interconnects WL[0 . . . y] herein are illustrated as being logicalequivalents with respect to each row of NAND strings. In an alternateembodiment, the decode module 18 can provide logically uniqueinterconnects to each NAND string pair. For example, the decode module18 can provide separate control signals WL[0] to each row of NANDstrings to reduce the amount of capacitance being charged during a readcycle. Alternatively, the decode module 18 can connect two or more NANDstrings to the same interconnect WL[0] to reduce the silicon area ofword line driver circuitry of decode module 18.

1. A method comprising: selecting a first NAND string of a firstplurality of NAND strings of a first NAND string module, the first NANDstring module comprising a second plurality of NAND strings and anoutput, the output electrically connected to a first global bit lineassociated with a sense amplifier; and communicating information at abit line of the first NAND string to the output of the first NAND stringmodule via a buffer in response to being selected.
 2. The method ofclaim 1, further comprising: selecting a second NAND string of the firstplurality of NAND strings subsequent to selecting the first NAND string;and communicating information at bit line of the second NAND string tothe output of the NAND string module via the buffer in response to thesecond NAND string being selected during the second read operation,where information at the bit line of the first NAND string is notcommunicated to the output of the NAND string module via the buffer wheninformation at the bit line of the second NAND string is beingcommunicated via the buffer.
 3. The method of claim 2, furthercomprising: selecting a third NAND string of the second plurality ofNAND strings subsequent to selecting the first NAND string; andcommunicating information at bit line of the third NAND string to theoutput of the first NAND string module via the buffer in response to thethird NAND string being selected, where information at the bit line ofthe first NAND string is not communicated to the output of the NANDstring module via the buffer when information at the bit line of thefirst or second NAND string is being communicated via the buffer.
 4. Themethod of claim 3, wherein the buffer is between the first plurality ofNAND strings and the second plurality of NAND strings.
 5. The method ofclaim 4, wherein the first plurality of NAND strings and the secondplurality of NAND strings comprise all of the NAND strings of the firstNAND string module.
 6. The method of claim 3 wherein: communicatinginformation at the bit line of the first NAND string to the output ofthe first NAND string module further comprises communicating theinformation at the bit line of the first NAND string to the output ofthe first NAND string module via a first multiplexer; and communicatinginformation at the bit line of the third NAND string of the secondplurality of NAND strings to the output of the NAND string modulefurther comprises communicating the information at the bit line of thesecond NAND string to the output of the first NAND string module via asecond multiplexer.
 7. The method of claim 2 wherein: communicatinginformation at the bit line of the first NAND string to the output ofthe first NAND string module further comprises communicating theinformation at the bit line of the first NAND string to the output ofthe first NAND string module via a first multiplexer; and communicatinginformation at the bit line of the second NAND string to the output ofthe NAND string module further comprises communicating the informationat the bit line of the second NAND string to the output of the firstNAND string module via the first multiplexer. ¶
 8. The method of claim1, wherein selecting the first NAND string is in response to the firstNAND string being selected during a read cycle, and information is notcommunicated between the bit line of the first NAND string and theoutput of the NAND string module via the buffer during a write cycle. 9.The method of claim 1, further comprising: biasing, during a read cycleof a NAND storage cell of the first NAND string, a second global bitline to a reference signal in response to selecting the first NANDstring, the reference signal having a value independent of a storagestate of the NAND storage cell; providing a first logic value to anoutput node during the read cycle in response to the reference signalwhen the NAND storage cell has a first storage state; and providing asecond logic value to the output node during the read access based uponthe reference signal when the NAND storage cell has a second storagestate.
 10. A device comprising: a sense amplifier comprising a firstinput and a second input; a global bit line to communicate informationto the first input of the sense amplifier; and a plurality of NANDstring modules, each one of the plurality of NAND string modulesrespectively comprising the following corresponding elements: a firstnode to communicate information to the global bit line, a buffer, aplurality of data paths, each data path of the plurality of data pathsrespectively comprising the first node, a corresponding NAND string of aplurality of NAND strings, and the buffer connected in series betweenthe corresponding NAND string and the first node to communicateinformation between the corresponding NAND string and the first node,wherein a first data path of the plurality of data paths comprises thefirst node, the buffer, and a first NAND string of the plurality of NANDstrings, and a second data path of the plurality of data paths comprisesthe first node, the buffer, and a second NAND string of the plurality ofNAND strings.
 11. The device of claim 10, wherein the buffer includes afirst portion of an active region and the first NAND string includes asecond portion of the active region.
 12. The device of claim 10, whereinthe buffer is an open drain buffer.
 13. The device of claim 10, whereineach one of the plurality of NAND string modules further respectivelycomprises: a first switch comprising a control node, a first dataelectrode coupled to an output of the buffer, and a second dataelectrode, the first switch being operable to communicate informationbetween the first data electrode and the second data electrode inresponse to an asserted read signal at the control node during a readaccess; and a second switch comprising a control node, a first dataelectrode coupled to the input of the buffer, and a second dataelectrode, the second switch being operable to communicate informationbetween the first data electrode and the second data electrode inresponse to an asserted write signal at the control node during a writeaccess.
 14. The device of claim 10, wherein each one of the plurality ofNAND string modules further respectively comprises: a first transistorcomprising a control input electrically connected to a second node, afirst current electrode, and a second current electrode, whereininformation at the output of the buffer is to be communicated to thefirst node via the first and second current electrodes of the firsttransistor.
 15. The device of claim 14, wherein each one of theplurality of NAND string modules further respectively comprises: asecond transistor comprising a control input electrically connected to athird node, a first current electrode, and a second current electrode,information at the input of the buffer to be communicated to the firstnode via the first and second current electrodes of the secondtransistor.
 16. The device of claim 10 further comprising: a resistivedevice comprising a first current electrode electrically connected tothe control input of the buffer, and a second electrode electricallycoupled to a reference node, the reference node operable to provide areference signal.
 17. The device of claim 16, wherein the resistivedevice includes a first portion of an active region and the NAND stringincludes a second portion of the active region.
 18. A device comprising:a global bit line; and a plurality of NAND string modules each one ofthe plurality of NAND string modules respectively comprising thefollowing corresponding elements a node electrically connected to theglobal bit line, a NAND string, and a buffer to communicate informationbetween the NAND string and the node.
 19. The device of claim 18,wherein the buffer is an open drain buffer connected in series betweenthe NAND string and the node to communicate information between the NANDstring and the node during a read operation.
 20. The device of claim 18wherein the buffer is to communicate the information between the NANDstring and the node during a first read operation, and to communicatebias information between a bias reference node and the node during asecond read operation.